The present invention relates to a membrane separation apparatus and to its use for the separation of fluid mixtures by, for example, pervaporation, vapor permeation and vacuum membrane distillation.
Pervaporation, vapor permeation, and vacuum membrane distillation (VMD), are all well known processes for separating fluid mixtures. In all of these three separation processes, an initial feed stream is separated into two streams: i) the retentate stream, which is rich in non-permeating components; and ii) the permeate stream, which is rich in the components which are able to pass through a selectively permeable membrane used in the separating process.
In pervaporation, the membrane acts as a selective barrier between the initial feed in liquid phase and the permeate in vapor phase. Thus, the membrane only allows the desired component(s) of the initial feed stream to transfer through the membrane by vaporization. The separation process is governed by the chemical affinity of the separating component(s) and the membrane, and not by the volatility difference of the separating components or the vapor liquid equilibrium. The driving force for the transfer of the permeating components(s) of the initial mixture is the partial pressure gradient of the selective or better permeating component(s) of the mixture across the membrane. Vapor permeation utilizes the same type of membrane. The only difference is that the initial feed stream, instead of being in the liquid phase as with pervaporation, is in contact with the membrane in the vapor phase.
Liquid mixtures with components having different volatilities and marked different boiling points are usually separated by distillation. However, in many cases during distillation, commonly used solvents, including aqueous mixtures, such as ethanol-water, isopropanol-water, etc. reach a limit point at which the concentration of the components in the vapor phases is similar to the concentration of the components in the liquid phase. This point is called the azeotropic point. Beyond the azeotropic point, simple distillation cannot perform any further separation. This problem is traditionally overcome by the addition of a third compound (solvent) that selectively associates with the more polar key components in the initial feed mixture, and can significantly increase the relative volatility of close-boiling-point components, thus making further separation by distillation beyond the azeotropic point technically possible. Although this method is widely used in industry, it suffers the drawback of contaminating the products by the third component, which can be especially detrimental in food or pharmaceutical applications.
Pervaporation and vapor permeation each offer an alternative solution to distillation in this regard. In these two processes, membranes are exposed to the initial mixed feed stream on one side of the membrane, and a vacuum is applied on the opposite other side thereof. One of the components of the initial feed stream is preferably absorbed by the membrane and diffuses through the membrane to be removed as vapor from the other side. The component of the initial feed stream that passes through the membrane is called the permeate. Since the separation is not dependent on the vapor liquid equilibrium, a proper selection of the membrane will allow separation of fluid mixtures both below and above the azeotropic point.
The VMD process is very similar to pervaporation. The main difference with pervaporation is that the membranes used therein, through which one (or more) of the components diffuse, are non-porous. In contrast, in VMD, separation takes place by evaporation through porous hydrophobic (water repellant) membranes. The hydrophobic nature of the membrane prevents the flow of water through its pores. As long as pressure on the vacuum side is maintained below the minimum required for liquid to penetrate the pores, a liquid-vapor interface exists at the pores of the membrane. Separation is thus governed by the vapor-liquid equilibrium.
Due to the porous nature of the membrane used in VMD, the permeate flux is significantly higher than achieved by pervaporation membranes Accordingly, VMD is particularly useful where the volatility of the components to be separated is quite different: eg. the removal of volatile organic compounds from water; evaporating liquid from a salt solution, for example, desalination of sea or brackish water for making ultra-pure water.
The flat sheet membranes used in all of the three processes discussed above can be housed in a module having the same general design. These modules should have reliable sealing between the feed side of the module and the permeate side of the module. They must also have high resistance to harsh operating conditions. To make these separation processes economically feasible, these modules should house a significantly large membrane surface area.
One way of reduction in overall size of the separation device is by the utilization of plate and frame type modules, each housing a plurality of membrane layers stacked with respect to one another in close parallel relation, and operatively connected for exposure to the fluid feed mixture to be separated. An example of such a prior art device is described in U.S. Pat. Nos. 5,437,796 (Bruschke et al.) and 4,769,140 (van Dijk et al.), which patents teach a plurality of feed plates, gaskets and membranes stacked upon one another.
However, the plate and frame module of Bruschke et al. and van Dijk et al. suffer from undue complexity of design utilizing a very large number of layers of components for securing and sealing the membranes, resulting in unduly high costs of production, installation and maintenance.
A further drawback of the separation devices of the prior art is the general way in which their membrane separation modules are housed for applying a vacuum thereacross. The known practice is to arrange the plurality of membrane separation modules within a single large vessel or housing (similar to a giant bell jar) which is kept under vacuum during operation. This greatly complicates maintenance of the plurality of separation modules within the vacuum vessel, as, over a period of time, the layers of gaskets in the separation modules begin to loosen, so as to require periodic tightening of the bolts or of the tide rods that hold the gaskets in sealing relation with the membranes. Failure to do this tightening maintenance on a regular basis may result in the gaskets leaking, with consequential loss of separating efficiency in the respective module. Following current practice, it is not possible to access a particular separation modules for tightening or other maintenance without first removing the common vacuum vessel. Such removal cannot be accomplished online, and requires shutting down the entire separation plant, at significant downtime cost. As a result, preventative maintenance, of the type just discussed is not carried out on a routine basis, but is typically left until the lack thereof causes a general plant shut down, with consequent removal of the vacuum vessel.
It should also be appreciated that actual lifting of the vacuum vessel typically requires a heavy duty crane, and, in some instances, such as where the separation plant is located inside of a building, removal of a portion of the ceiling or roof of the building. Furthermore, if any one of the membrane separation modules fails, or otherwise requires maintenance, the entire vacuum vessel containing the plurality of separation modules must be shut down to identify, and service, the particular malfunctioning module. This is both inconvenient and costly.
It is an object of the present invention to overcome, inter alia, the shortcomings of the prior art described above by providing a separation apparatus that is suitable for use for pervaporation, vapor permeation and vacuum membrane distillation that does not suffer from unduly high production, installation or maintenance costs and undue complexity of assembly.
According to a first aspect of the invention there is provided an apparatus for the separation of a fluid mixture into a permeate and a retentate. This apparatus comprises a separation module defining a substantially horizontal primary axis. The separation module comprises a fluid mixture inlet, a retentate outlet and a permeate outlet. A pair of substantially planar, horizontally extending fluid separation membranes, each pair comprising a first fluid separation membrane and a second fluid separation membrane, with each membrane having an active surface and an oppositely facing secondary surface, with the active surfaces facing one another. A fluid containment gasket is interposed between the pair of fluid separation membranes in sealing engagement with the active surfaces of both membranes of the pair, so as to form, in combination with said first and said second fluid separation membranes, a fluid containment chamber, with the first fluid separation membrane forming a lower end of said fluid containment chamber and the second fluid separation membrane forming an upper end of said fluid containment chamber. The fluid containment gasket has an inlet wall normal to the primary axis, an outlet wall substantially parallel to the inlet wall and axially spaced therefrom, and two opposed sidewalls extending between said inlet wall and the outlet wall in substantially parallel relation to the primary axis. The fluid containment chamber also has an inlet area adjacent the inlet wall, and a outlet area adjacent the outlet wall. A pair of substantially planar, horizontally extending membrane support plates are also provided. Each such pair comprises a first membrane support plate and a second membrane support plate vertically spaced from the first membrane support plate. Each membrane support plate has an upper and a lower surface, a central body portion and a substantially quadrilateral outer perimeter portion having one or more lateral edge portions. Each membrane support plate also has defined therethrough a plurality of elongate horizontal channels, with each elongate channel extending between the central body portion of the membrane support plate to a terminus of the elongate channel located in at least one of the lateral edge portions of the membrane support plate. The secondary surface of the first separation membrane is positioned within the separation module in adjacent overlying relation to the upper surface of the central body portion of the first membrane support plate. The secondary surface of the second separation membrane is positioned in the separation module in adjacent underlying relation to the lower surface of the central body portion of the second membrane support plate. A fluid delivery means provides for egress of the fluid mixture from the fluid mixture inlet into the inlet area of each of said fluid containment chambers. A permeate collecting means provides for collection of the permeate from the terminus of each of the plurality of elongate channels for delivery thereof to the permeate outlet. A retentate collection means provides for collection of the retentate from the outlet area of each of the fluid containment chambers for delivery thereof to the retentate outlet. A feeding means such as a centrifugal pump, feeds the fluid mixture to be separated to the fluid mixture inlet of the separation module.
According to another aspect of the present invention, there is advantageously provided a means for the creation of a negative pressure differential between the fluid containment chamber and the elongate channels. Such pressure differential assists permeation of the permeate across the first and second fluid separation membranes from the fluid containment chamber into the elongate channels of the central body portion of the first and second membrane support plates, for subsequent collection by the permeate collecting means for delivery to the permeate outlet.
According to yet another aspect of the present invention, there is preferably provided a plurality of separation modules and a selector means. The selector means is adapted for the selective configuration of the separation modules into a series configuration, wherein said separation modules are fluidly connected to one another in sequence such that the fluid mixture is fed by the feeding means into the fluid mixture inlet of a first of the separation modules, with the retentate outlet of each of the separation modules being in fluid communication with the fluid mixture inlet of the next of said separation modules in said sequence, and for the selective configuration of said separation modules in a parallel configuration, wherein the feeding means feeds said fluid mixture to the fluid mixture inlet of each of the separation modules for contemporaneous separation of the fluid mixture in each of said separation modules.
According to one embodiment of the present invention, the fluid delivery means comprises a feed distributor positioned within each of said fluid containment chambers adjacent the respective fluid inlet area thereof. The feed distributor has a substantially cylindrical inner bore centred about a substantially vertical fluid inlet axis. A substantially cylindrical outer sidewall is concentrically spaced from said inner bore. A substantially horizontal, planar, annular upper surface extends between said inner bore and said outer sidewall. A substantially horizontal, planar, annular lower surface, spaced apart from the annular upper surface along the vertical axis, extends between the inner bore and the outer sidewall. A plurality of fluid transmission channels are provided withing each feed distributor, with radially extend between the inner bore and the outer sidewall thereof substantially towards the respective outlet wall of the fluid containment chamber within which the feed distributor is positioned.
According to yet another aspect of the present invention, the retentate collection means comprises a retentate receiver positioned within each of said fluid containment chambers adjacent the respective outlet area thereof. The retentate receiver has a substantially cylindrical inner bore surrounding a substantially vertical fluid outlet axis and a substantially cylindrical outer sidewall concentrically spaced from the inner bore. A substantially horizontal, planar, annular upper surface extends between the inner bore and the outer sidewall, and a substantially horizontal, planar, annular lower surface spaced apart from the annular upper surface along said fluid outlet axis extends between the inner bore and the outer sidewall. A plurality of fluid transmission channels radially extend between the inner bore and the outer sidewall substantially towards the respective outlet wall of the fluid containment chamber withing which the retentate receiver is positioned.
According to yet another aspect of the invention, the separation module preferably defines a substantially horizontal primary axis, a substantially vertical fluid inlet axis and a fluid outlet axis substantially parallel to the fluid inlet axis and spaced apart therefrom along the primary axis, and comprises a plurality of pairs of substantially planar, horizontally extending fluid separation membranes, with each pair of separation membranes comprising a first fluid separation membrane and a second fluid separation membrane with each fluid separation membrane having an active surface and an oppositely facing secondary surface, with the active surfaces facing one another. An equal plurality of fluid containment gaskets are interposed one each between each pair of fluid separation membranes in sealing engagement with the active surfaces of both membranes of the respective pair so as to form, in combination with the first and second fluid separation membranes of the respective pair, an equal plurality of fluid containment chambers. The first fluid separation membrane of each pair forms a lower end of the respective fluid containment chamber and the second fluid separation membrane of each pair forms an upper end of the respective fluid containment chamber. The fluid containment gaskets each have, respectively, an inlet wall normal to the primary axis and adjacent to the outlet axis, an outlet wall, adjacent to the outlet axis, substantially parallel to the inlet wall and spaced therefrom along the primary axis, and two opposed sidewalls extending between the inlet wall and the outlet wall in substantially parallel relation to the primary axis. Each of the fluid containment chambers also has a respective inlet area thereof adjacent the inlet wall thereof and a respective outlet area adjacent the outlet wall thereof. A second plurality of substantially planar, horizontally extending membrane support plates is provided, comprising one or more first membrane support plates and one or more second membrane support plates. The first membrane support plates are vertically spaced from the second membrane support plates, and each membrane support plate has a central body portion, and a substantially quadrilateral outer perimeter portion having one or more lateral edge portions The central body portion further has an upper surface, a lower surface and each of the membrane support plates further has defined therethrough a plurality of elongate horizontal channels, with each elongate channel extending between the central body portion of the respective membrane support plate to a terminus of the each elongate channel located in one or more of the edge portions of the respective membrane support plate. The plurality of fluid containment chambers are disposed in vertically-stacked relation within the separation module, with the membrane support plates being interleaved one each in alternating order between the fluid containment chambers. Each of the first separation membranes is positioned in adjacent overlying relation to the upper surface of the central body portion of each first membrane support plate, with the secondary surface of each second separation membrane being positioned in the separation module in adjacent underlying relation to the lower surface of the central body portion of each second membrane support plate, such that the termini of the elongate channels extend laterally beyond the lateral extent of the fluid containment chambers A fluid delivery means for egress of the fluid mixture from the fluid mixture inlet into the inlet area of each of the fluid containment chambers is also provided. A permeate collecting means for collection of the permeate from the terminus of each of the plurality of elongate channels for delivery to the permeate outlet is also provided, as is a retentate collection means for collection of the retentate from the outlet area of each of the fluid containment chambers for delivery to the retentate outlet. Lastly, a feeding means for selectively feeding the fluid mixture to the fluid mixture inlet of the separation module is provided, preferably in the form of a centrifugal pump.
According to another aspect of the present intention, the separation module further comprises an equal plurality of pairs of membrane spacer gaskets, with each such pair of membrane spacer gaskets having a first membrane spacer gasket and a second membrane spacer gasket, each of said first membrane spacer gaskets being disposed between a respective first fluid separation membrane and a respective first membrane support plate so as to hold the respective first fluid separation membrane in spaced relation from said the respective membrane support plate, and wherein each of the second membrane spacer gaskets is disposed between a respective second fluid separation membrane and a respective second membrane support plate to hold the respective second fluid separation membrane in spaced relation from the respective second membrane support plate.
According to yet another aspect of the present intention, the separation module further comprises a second plurality of pairs of spacer meshes with one of the spacer meshes of each pair of spacer meshes being held in contacted relation against the lower surface of the central body portion of each of the membrane supporting plates by a respective one of the second spacer gaskets and with the other one of the spacer meshes of the each pair of spacer meshes being held in contacted relation against the upper surface of the central body portion of each of the membrane supporting plates by a respective one of the first spacer gaskets.
According to yet another aspect of the present invention, the permeate collecting means preferably comprises a shroud member sealingly extending between the base plate and the top plate to form, in combination with the base plate and the top plate, a housing for the membrane support plates, the fluid separation membranes, the spacer meshes, the membrane spacer gaskets, the fluid containment gaskets, the retentate receivers and the feed distributors.
According to yet another aspect of the present invention, there is additionally provided a holding means for immobilizing the membrane support plates, the base plate, the top plate, the fluid separation membranes, the spacer meshes, the membrane spacer gaskets, the fluid containment gaskets, the retentate receivers and the feed distributors, the holding means preferably comprising a plurality of threaded rods rigidly connected to the base plate and extending upwardly therefrom within the shroud member and thence through a corresponding plurality of bores in the top plate; and a corresponding plurality of nuts adjustably, releasably connected to the threaded rods above the top plate, each of the nuts preferably comprising a dome nut with an integral o-ring seal partially projecting from a bottom face thereof for sealing contact with the top plate.
Other advantages, features and characteristics of the present invention, as well as methods of operation and functions of the related elements of the structure, and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following detailed description and the appended claims with reference to the accompanying drawings, the latter of which is briefly described hereinbelow.